The installation of optical fiber aerial cables in their medium and high voltage paths makes it possible to connect communications of power supply plants, all metropolitan centers of a region as well as their larger cities and communities while circumventing the networks of governmental and/or private telecommunications companies.
For this purpose, self-supporting, i.e., self-terminating aerial cables and so-called ADL cables (All-Dielectric Lash) are mainly used. While self-supporting aerial cables are provided with non-metallic tension relief elements (All-Dielectric-Self-Supporting or ADSS cable) or with a messenger wire (Figure-8-cable) which absorbs the tensile forces, the ADL cables have no supporting elements. Normally they are fastened to an already installed ground or phase wire of the high voltage line by means of a glue or lashing strip. An overview concerning optical aerial cables can be found in xe2x80x9cLichtwellenleiterkabelxe2x80x9d by G. Mahlke, P. Goessing, Publicis MCD Verlag, 5th edition, pp. 157-162 (1998).
As ground and phase wires, the ADSS cable stretched between the poles of a high voltage line, is subject to considerable mechanical stresses due to wind, icing, or large temperature variations. Besides the necessary tensile strength, the cable manufacturer has to consider the electrical stress mechanisms which lead to accelerated aging and finally to damage or destruction of the cable jacket, especially corona discharge in the area of the anchoring spirals and the so-called tracking effect (dry-band-arcing), by using appropriate measures and special jacketing materials.
To suppress these effects the following suggestions are made:
to manufacture the jacket of the aerial cable from a plastic which is self-quenching and stable against partial discharge and non-tracking or supply it with a tracking resistant outer layer (see U.S. Pat. No. 4,673,247 and WO 99/04300),
to soak the aramid yarns serving as tension-proof elements with a low conducting liquid (see U.S. Pat. No. 4,776,665), and
to arrange electrical conductor elements, not connected to each other and partially overlapping in the axial direction, between the cable core and the outer jacket (see EP 0 695 431 B1).
In U.S. Pat. No. 5,563,976 it is suggested, to divert the charges induced by capacitive charging of the ADSS cable to the pole by means of several carbon containing fiber strands which surround the cable core in a spiral or helix fashion. The fiber strands with a carbon content of between 65% and 92% of weight, are produced by pyrolysis of polyacrylonnitrile. Additionally, the distribution of the fiber strands influencing the voltage path along the cable cannot sufficiently be adjusted to the given conditions or standards.
The objective of the invention is an aerial cable with multiple uses, especially a dielectric optical communications cable capable of being installed in the field of a high voltage line, whose jacket has high longevity. Additionally the manufacture of the cable should be simple, variable and cost effective.
These requirements are met by an aerial cable which contains: at least one optical transmission element surrounded by a protective covering, at least one tension-proof element, an outer jacket and several strand- or string-like elements completely embedded into the jacket material, where the strand- or string-like elements consist of a polymer with semiconductor characteristics or of a polymer mixture with semiconductor characteristics and where the resistance of the polymers or the polymer mixture are in the range of 105 xcexa9/m and 109 xcexa9/m.
The dependent patent claims concern preferred constructions and further development of the aerial cable. A process for the manufacture of such a cable provides for a cable core with at least one optical transmission element and an outer jacket where several fusible strands or fibers made from a polymer having semiconductor characteristics or a polymer mixture having semiconductor characteristics produced by means of coextrusion are embedded into the fusible jacket material.
In an aerial cable constructed according to the invention, the induced electrical charges are discharged by means of semiconductor fusible fibers completely embedded within the PE jacket to the grounded anchoring spiral fastened to the pole. Even on the jacket surface, which is still wet and only partially dried off, no large potential differences in the longitudinal direction of the cable can occur. Additionally, the fusible fibers can be embedded into the jacket material in a simple manner, in the desired numbers, with the required thickness and with nominal radial distance to the longitudinal axis of the cable, running exactly parallel to the longitudinal axis of the cable. This last factor contributes to the homogenization of the field strength distribution along the cable.